1. Field of the Invention
This invention relates to detecting faulty components in a heating system. More particularly, this invention relates to detecting a faulty combustion air switch in a heating system.
2. Description of the Prior Art
Forced air industrial heating systems, such as furnaces, ovens and boilers, typically have a blower or burner motor which forces air into a combustion chamber. There the air is combined with fuel and ignited. If not enough combustion air is blown into the combustion chamber, a fuel-rich environment results in the combustion chamber. The environment may become so fuel-rich that there is not enough air in the combustion chamber to support combustion. If insufficient combustion air is detected, uncombusted fuel and products of combustion will continue to build up in the combustion chamber; possibly creating an explosive, hazardous condition.
Therefore, it is necessary to have some type of combustion air detector for detecting whether sufficient combustion air flow is present for safe burner operation. Combustion air detector switches can be of several types including, among others, air pressure switches and sail switches.
In any case, these combustion air detectors are generally transducers which control a set of switch contacts which are wired to directly control power to fuel valves which supply fuel to the combustion chamber in the heating system. Therefore, when insufficient combustion air for safe, clean combustion is detected, power to the fuel valves is interrupted causing them to close. If the switch contacts controlled by the combustion air detection transducer are short-circuited, a loss of sufficient combustion air in the combustion chamber may never be detected and a fuel-rich, possibly hazardous environment may arise in the combustion chamber.
Heating system controllers sold in the United States are not required to test for the ability of the combustion air transducer controlled contacts to open. Therefore, only when a heating system operator specifically does so, are the contacts tested.
In Europe, on the other hand, the combustion air contacts are required to be tested. This test has typically required the contacts to have a single-pole double-throw (SPDT) construction. The normally closed contacts are tested prior to, or at the start of, a heating system's cycle for closure. If they are not closed, the heating system is not permitted to start. Then, when the combustion air detector is detecting adequate air in the combustion chamber, the normally open contacts are tested for closure. If these contacts are open, the heating system will be recycled or shut down.
Although this test does typically detect short-circuited contacts, the SPDT contacts add cost to the heating system. They add cost to the combustion air detection switch, to the controller controlling the heating system, and to the field wiring required for installation.
Another method which has been used to test for the ability of the combustion air contacts to open is to designate a specific input terminal to the controller to monitor the contacts. However, this method also adds cost to the controller and takes up controller inputs.
Therefore, there is a need for a method and apparatus which tests for the ability of combustion air contacts to open without requiring the contacts to be of SPDT construction or to have a dedicated input terminal to the controller.